Light emitting device

ABSTRACT

It is an object of the present invention to provide a light emitting device which is less affected by a malfunction caused in a light emitting element. It is another object of the invention to provide a light emitting device in which light emitting elements are connected in series. As to a light emitting device of the invention, groups of circuits each having a light emitting element and a limiter are connected in parallel. Here, a light emitting element and a limiter are connected in series. The number of the circuits may be at least two or more. Further, each circuit group includes at least one light emitting element.

TECHNICAL FIELD

The present invention relates to a light emitting device, particularlyto a light emitting device including a light emitting element having alight emitting layer between a pair of electrodes.

BACKGROUND ART

A light emitting element having a light emitting layer between a pair ofelectrodes is in actual use as a pixel for operating a display device.In recent years, such a light emitting element attracts attention as alight source of a lighting device as well as a display device.

As to a lighting device, sophistication as required for a display deviceis not particularly required. However, the influence on a lightingdevice caused due to a defect of one light emitting element is greaterinstead. Specifically, a malfunction in which a light emitting elementdoes not emit light, the illuminance is extremely reduced due to ashort-circuit of a light emitting element, or the like can be caused.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a light emittingdevice which is less affected by a malfunction caused in a lightemitting element. It is another object of the invention to provide alight emitting device in which light emitting elements are connected inseries.

As to a light emitting device of the invention, groups of circuits eachhaving a light emitting element and a limiter are connected in parallel.Here, a light emitting element and a limiter are connected in series.The number of the circuits may be at least two or more. Further, eachcircuit group includes at least one light emitting element.

In the invention, there is not particular limitation on the number oflimiters. Accordingly, one circuit group includes at least one limiter.Further, the limiter may be provided closer to either a high potentialpower supply or a low potential power supply; however, the limiter maypreferably be provided on a side where current flows into a circuitgroup including a light emitting element and the limiter (namely, thelimiter is preferably provided closer to a high potential power supplythan the light emitting element).

Here, a limiter comprises a circuit having an element or a combinationof plural elements, which is used for controlling excess current flowninto a light emitting element.

A light emitting device according to the invention includes a firstlight emitting element and a second light emitting element. The firstlight emitting element and the second light emitting element eachincludes a light emitting layer between a first electrode and a secondelectrode. The first electrode included in the first light emittingelement and the second electrode included in the second light emittingelement are overlapped and electrically connected.

According to the invention, a malfunction of a light emitting devicecaused due to short circuit between electrodes in a light emittingelement can be reduced. Further, a light emitting device including lightemitting elements connected in series, which can be manufactured easilycan be obtained according to the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a figure showing a mode of a light emitting device accordingto the present invention.

FIG. 2 is a figure showing a mode of a light emitting element in a lightemitting device according to the present invention.

FIG. 3 is a figure showing a mode of a light emitting element in a lightemitting device according to the present invention.

FIG. 4 is a figure showing a mode of a light emitting element in a lightemitting device according to the present invention.

FIG. 5 is a figure showing a mode of a light emitting element in a lightemitting device according to the present invention.

FIG. 6 is a figure showing a mode of a light emitting device accordingto the present invention.

FIGS. 7A and 7B are figures showing a mode of a light emitting deviceaccording to the present invention.

FIG. 8 is a figure showing a mode of a light emitting device accordingto the present invention.

FIGS. 9A to 9C are figures showing a mode of an electronic devise usinga light emitting device according to the present invention.

FIG. 10A and FIG. 10B are figures showing a mode of a light emittingdevice according to the present invention.

FIG. 11 is a figure showing a mode of a light emitting device accordingto the present invention.

FIG. 12 is a figure showing a mode of a light emitting device accordingto the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment modes of the invention will be hereinafter described indetail with reference to drawings. However, it is easily understood bythose skilled in the art that the invention can be implemented in manydifferent modes and various changes may be made in forms and detailswithout departing from the spirit and the scope of the invention.Therefore, the invention should not be limited by the descriptions ofembodiment modes below.

Embodiment Mode 1

A mode of a light emitting device of the invention will be describedwith reference to FIG. 1.

In FIG. 1, a plurality of groups of circuits 121 each including lightemitting elements 101 a to 101 d and a limiter 111 are connected inparallel.

In each of the circuits 121, the light emitting elements 101 a to 101 dand the limiter 111 are respectively connected in series. In FIG. 1,four light emitting elements are included in each of the circuits 121;however, the number of light emitting elements in a circuit is notlimited in particular, and a circuit may include at least one lightemitting element. Here, the light emitting elements 101 a to 101 d eachhas a light emitting layer 133 between a pair of electrodes (a firstelectrode 131 and a second electrode 132) as shown in FIG. 2.

The limiter 111 is not either limited in particular; it may be any onewhich can control to prevent excess current flow. For example, thelimiter may be composed of one transistor or may be a circuit of acombination of plural elements such as a transistor and a diode.

As the limiter 111, for example, the structure shown in FIG. 10A, FIG.10B, FIG. 11 or FIG. 12 can be used. However, the structure is notlimited to this. FIG. 10A shows a limiter 111 a comprising a transistor301 and a resistor 302. Alternatively, the limiter 111 may compriseeither any one of the transistor 301 or the resistor 302 as with alimiter 111 b shown in FIG. 10B or a limiter 111 c shown in FIG. 11.Here, a gate electrode of the transistor 301 may be connected to a node127 as shown in FIG. 10A; otherwise the gate electrode of the transistor301 may be connected to a node 128 as shown in FIG. 10B, for example. Inthe case where the limiter 111 a and the limiter 111 b are providedcloser to a high potential power supply as shown in FIG. 10A and FIG.10B, respectively, the transistor 301 may preferably be a p-channeltransistor. In the case where the limiter 111 c is provided closer to alow potential power supply as shown in FIG. 11, the transistor maypreferably be an n-channel transistor 303. Further, the limiter 111 maybe a limiter 111 d comprising a diode as shown in FIG. 12.

An end of each of the circuits 121 is connected to a wiring 122 at anode 125, and the other end is connected to a wiring 123 at a node 126.The wirings 122 and 123 are connected to a power supply 124.

Voltage is applied to the light emitting elements 101 a to 101 d fromthe power supply 124 via the wirings 122 and 123. Potential differencesare generated between electrodes in respective light emitting elements101 a to 101 d; thus, current flows. After a luminescent material israised to an excited state due to the flowing current, light is emittedin returning to the ground state. The first electrodes 131 are eachconnected to the power supply via wirings to be applied with potentialhaving the same polarity. Further, the second electrodes 132 are eachconnected to the power supply via wirings to be applied with potentialhaving a polarity opposite to the first electrodes. Here, there is noparticular limitation on whichever polarity of potential is applied toeach electrode.

As to such a light emitting device, although a malfunction such as ashort circuit is caused between electrodes in any one of the lightemitting elements, good operation can be conducted without placing asignificant burden on other light emitting elements.

For example, even in the case where the electrodes of the light emittingelement 101 b are short-circuited, the other light emitting elements(light emitting elements 101 a, 101 c, and 101 d) can emit light sincethey are connected in series with each other. Further, even in the casewhere most or all of the light emitting elements included in any one ofthe plurality of circuits 121 is short-circuited, excess current doesnot flow since each of them has the limiter 111, so that current supplyto other circuits is not prevented.

As to a light emitting device according to the invention, the firstelectrode 131 and the second electrode 132 that are included in thelight emitting element are not particularly limited; however, it ispreferable that at least one of the pair of electrodes can transmitvisible light. Thus, light can be emitted from one or both sides of thelight emitting device.

Further, the light emitting layer 133 is not either limited inparticular. Either or both an organic compound and an inorganic compoundare contained in the light emitting layer 133. Further, the lightemitting layer 133 may have either a single layer structure or amultilayer structure. In the case of a multilayer structure, luminescentmaterials having different luminescent colors are made emit light inrespective layers, so that the colors are visually recognized to bemixed.

Embodiment Mode 2

In this embodiment mode, a mode of a light emitting element included ina light emitting device according to the present invention will bedescribed with reference to FIG. 3.

FIG. 3 shows a light emitting element that has a light emitting layer213 between a first electrode 201 and a second electrode 202. In thislight emitting element, a hole injected from the first electrode 201 andan electron injected from the second electrode 202 are recombined in thelight emitting layer 213 to bring a luminescent material to an excitedstate. Here, a luminescent material is a substance which can emit lightwith a desired emission wavelength and has good luminous efficiency.Then, light is emitted when the luminescent material in the excitedstate returns to the ground state. It is to be noted that the firstelectrode 201 and the second electrode 202 respectively serve as ananode and a cathode in the light emitting element in the presentembodiment mode.

Here, the light emitting layer 213 is not particularly limited. However,it is preferable that the light emitting layer 213 be a layer in whichthe luminescent material is included so as to be dispersed in a layerformed of a material that has a larger energy gap than the luminescentmaterial has. This makes it possible to prevent quenching ofluminescence from the luminescent material due to the concentration. Itis to be noted that an energy gap indicates the energy gap between aLUMO level and a HOMO level.

The luminescent material is not particularly limited. A substance whichcan emit light with a desired emission wavelength and has good luminousefficiency may be selected and used as the luminescent material. Forexample, in obtaining red tinged luminescence, a substance whichexhibits luminescence with a peak of emission spectrum at 600 nm to 680nm, such as4-dicyanomethylene-2-isopropyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbreviation: DCJTI),4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbreviation: DCJT),4-dicyanomethylene-2-tert-butyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbreviation: DCJTB), periflanthene, or2,5-dicyano-1,4-bis-[2-(10-methoxy-1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]benzenecan be used. In the case of obtaining green tinged luminescence, asubstance which exhibits luminescence with a peak of emission spectrumat 500 nm to 550 nm, such as N,N′-dimethylquinacridone (abbreviation:DMQd), coumarin 6, coumarin 545T, or tris(8-quinolinolato)aluminum(abbreviation: Alq₃) can be used. Further, in the case of obtaining bluetinged luminescence, a substance which exhibits luminescence with a peakof emission spectrum at 420 nm to 500 nm, such as9,10-bis(2-naphthyl)-tert-butylanthracene (abbreviation: t-BuDNA),9,9′-bianthryl 9,10-diphenylanthracene (abbreviation: DPA),9,10-bis(2-naphthyl)anthracene (abbreviation: DNA),bis(2-methyl-8-quinolinolato)-4-phenylphenolate-gallium (abbreviation:BGaq), or bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum(abbreviation: BAlq) can be used. In addition to the above fluorescentsubstances, a phosphorescent substance such astris(2-phenylpyridine)iridium may be used.

The material used for dispersing the luminescent material is not limitedin particular. For example, carbazole derivatives such as4,4′-bis(N-carbazolyl)-biphenyl (abbreviation: CBP) and4,4′,4″-tris(N-carbazolyl)-triphenylamine (abbreviation: TCTA) and metalcomplexes such as bis[2-(2-hydroxyphenyl)-pyridinato]zinc (abbreviation:Znpp₂), bis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (abbreviation:Zn(BOX)₂), and tris(8-quinolinolato)aluminum (abbreviation: Alq₃) arepreferable in addition to compounds having an arylamine skeleton such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (abbreviation: α-NPD).

Although the first electrode 201 is not particularly limited, it ispreferable that the first electrode 201 be formed by using a materialthat has a larger work function when the first electrode 201 functionsas an anode as in the present embodiment mode. Specifically, in additionto indium tin oxide (ITO), indium tin oxide containing silicon oxide,and indium oxide including zinc oxide at 2 to 20%, gold (Au), platinum(Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron(Fe), cobalt (Co), copper (Cu), palladium (Pd), and the like can beused. The first electrode 201 can be formed by, for example, sputteringor vapor deposition.

Further, although the second electrode 202 is not particularly limited,it is preferable that the second electrode 202 be formed by using amaterial that has a smaller work function when the second electrode 202functions as a cathode as in the present embodiment mode. Specifically,aluminum containing an alkali metal or an alkali-earth metal such aslithium (Li) or magnesium, and the like can be used. The secondelectrode 202 can be formed by, for example, sputtering or vapordeposition.

In order to extract emitted light to the outside, it is preferable thatany one or both the first electrode 201 and the second electrode 202 bean electrode containing a material such as indium tin oxide that cantransmit visible light or an electrode formed to a thickness of severalto several tens nm so as to transmit visible light.

In addition, a hole transport layer 212 may be provided between thefirst electrode 201 and the light emitting layer 213 as shown in FIG. 3.Here, a hole transport layer is a layer that has a function oftransporting holes injected from an electrode to a light emitting layer.The hole transport layer 212 is provided to keep the first electrode 201away from the light emitting layer 213 in this way; thus, quenching ofluminescence due to a metal can be prevented.

The hole transport layer 212 is not particularly limited, and it ispossible to use a layer formed with the use of, for example, an aromaticamine compound (that is, compound including a bond of a benzenering-nitrogen) such as 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl(abbreviation: α-NPD),4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl (abbreviation:TPD), 4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine (abbreviation:TDATA), or4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenylamine(abbreviation: MTDATA). In addition, the hole transport layer 212 may bea layer that has a multilayer structure formed by combining two or morelayers each including the material mentioned above.

Further, an electron transport layer 214 may be provided between thesecond electrode 202 and the light emitting layer 213 as shown in FIG.3. Here, an electron transport layer is a layer that has a function oftransporting electrons injected from an electrode to a light emittinglayer. The electron transport layer 214 is provided to keep the secondelectrode 202 away from the light emitting layer 213 in this way; thus,quenching of luminescence due to a metal can be prevented.

The electron transport layer 214 is not particularly limited, and it ispossible to use a layer formed using, for example, a metal complexincluding a quinoline skeleton or a benzoquinoline skeleton, such astris(8-quinolinolato)aluminum (abbreviation: Alq₃),tris(5-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃), bis(10hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation:BAlq). In addition, a layer formed using, for example, a metal complexincluding a oxazole-based ligand or a thiazole-based ligand such asbis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (abbreviation: Zn(BTZ)₂),may be used. Further, a layer formed with the use of2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviation: to as OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen),bathocuproin (abbreviation: BCP), or the like may be used. In addition,the electron transport layer 214 may be a layer that has a multilayerstructure formed by combining two or more layers each including thematerial mentioned above.

Further, a hole injection layer 211 may be provided between the firstelectrode 201 and the hole transport layer 212 as shown in FIG. 3. Here,a hole injection layer is a layer that has a function of assistinginjection of holes from an electrode serving as an anode to a holetransport layer. It is to be noted that injection of holes into a lightemitting layer may be assisted by providing a hole injection layerbetween an electrode serving as an anode and the light emitting layerwhen no hole transport layer is particularly provided.

The hole injection layer 211 is not particularly limited, and it ispossible to use a layer formed using, for example, a metal oxide such asmolybdenum oxide (MoO_(x) vanadium oxide (VO_(x)), ruthenium oxide(RuO_(x)), tungsten oxide (WO_(x)), manganese oxide (MnO_(x)) Inaddition, the hole injection layer 211 can be formed using aphthalocyanine compound such as phthalocyanine (abbreviation: H₂Pc) orcopper phthalocyanine (abbreviation: CuPc), a high polymer such as apoly(ethylenedioxythiophene)/poly(styrene sulfonate) aqueous solution(PEDOT/PSS), or the like.

Further, an electron injection layer 215 may be provided between thesecond electrode 202 and the electron transport layer 214 as shown inFIG. 3. Here, an electron injection layer is a layer that has a functionof assisting injection of electrons from an electrode serving as acathode to the electron transport layer 214. It is to be noted thatinjection of electrons into a light emitting layer may be assisted byproviding an electron injection layer between an electrode serving as acathode and the light emitting layer when no electron transport layer isparticularly provided.

The electron injection layer 215 is not particularly limited, it ispossible to use a layer formed with the use of, for example, a compoundof an alkali metal or an alkali-earth metal such as lithium fluoride(LiF), cesium fluoride (CsF), or calcium fluoride (CaF₂). In addition, alayer in which a highly electron transportable material such as Alq₃ or4,4-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOS) is mixedwith an alkali metal or an alkali-earth metal such as magnesium orlithium can also be used as the electron injection layer 215.

In the above-described light emitting element according to theembodiment mode, each of the hole injection layer 211, the holetransport layer 212, the light emitting layer 213, the electrontransport layer 214, and the electron injection layer 215 may be formedby any method, for example, vapor deposition, inkjet, or coating. Inaddition, the first electrode 201 and the second electrode 202 may beformed by any method, for example, sputtering or vapor deposition.

In the above-described light emitting element, since the sum of thethicknesses of the layers provided between the first electrode 201 andthe second electrode 202 is as extremely thin as several tens nm toseveral hundreds nm; therefore, the first electrode 201 and the secondelectrode 202 can be short circuited. However, by applying the presentinvention, drop-off in illuminance or the like in the light emittingdevice can be reduced and the light emitting device can be operatedfavorably even when electrodes included in the light emitting elementare short-circuited.

Embodiment Mode 3

The mode of a light emitting element provided in a light emitting deviceaccording to the present invention is not limited to the one describedin Embodiment Mode 2. For example, the light emitting element may have aplurality of light emitting layers. For example, white light can beobtained by providing a plurality of light emitting layers and mixingluminescence from respective light emitting layers. In the presentembodiment mode, modes of light emitting elements each having aplurality of light emitting layers will be described with reference toFIGS. 4 and 5.

In FIG. 4, a first light emitting layer 763 and a second light emittinglayer 765 are provided between a first electrode 751 and a secondelectrode 752. It is preferable to provide a partition layer 764 betweenthe first light emitting layer 763 and the second light emitting layer765.

When voltage is applied so that the potential of the second electrode752 is higher than the potential of the first electrode 751, currentflows between the first electrode 751 and the second electrode 752, anda hole and an electron are recombined in the first light emitting layer763, the second light emitting layer 765, or the partition layer 764.Generated excitation energy transfers to both the first light emittinglayer 763 and the second light emitting layer 765 through the partitionlayer 764, and a first luminescent material contained in the first lightemitting layer 763 and a second luminescent material included in thesecond light emitting layer 765 are excited. Then, the excited first andsecond luminescent materials emit light in returning to the respectiveground states.

The first light emitting layer 763 includes a luminescent materialtypified by a fluorescent material such as perylene,2,5,8,11-tetra-tert-butylperylene (abbreviation: TBP),4,4′-bis(2-diphenylvinyl)biphenyl (abbreviation: DPVBi),4,4′-bis[2-(N-ethylcarbazole-3-yl)vinyl]biphenyl (abbreviation: BCzVBi),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation:BAlq), or bis(2-methyl-8-quinolinolato)-chlorogallium (Gamq₂Cl), or aphosphorescent material such asbis[2-(3,5-bis(trifluoromethyl)phenyl)pyridinato-N,C^(2′)]iridium(III)picolinate(abbreviation: Ir(CF₃ ppy)₂(pic)),bis[2-(4,6-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate(abbreviation: FIr(acac)), or bis[2-(4,6-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)picolinate (abbreviation: FIr(pic)),from which luminescence with a peak at 450 to 510 nm in an emissionspectrum can be obtained. In addition, the second light emitting layer765 includes a luminescent material serving as a luminescent material,from which luminescence with a peak at 600 to 680 nm in an emissionspectrum as in Embodiment Mode 2 can be obtained. Then, the luminescentcolor of luminescence from the first light emitting layer 763 and theluminescent color of luminescence from the second light emitting layer765 are emitted to the outside through one or both of the firstelectrode 751 and the second electrode 752. Luminescence emitted to theoutside is visually mixed to be visually recognized as white light.

It is preferable that the first light emitting layer 763 be a layer inwhich a luminescent material that is capable of exhibiting luminescenceof 450 to 510 nm is dispersedly included to be in a layer composed of amaterial (first host) that has a larger energy gap than the luminescentmaterial, or a layer composed of a luminescent material that is capableof exhibiting luminescence of 450 to 510 nm. As the first host, inaddition to α-NPD, CBP, TCTA, Znpp₂, and Zn(BOX)₂ mentioned above,9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-di(2-naphthyl)-2-tert-butylanthracene (abbreviation: t-BuDNA), andthe like can be used. Further, it is preferable that the second lightemitting layer 765 be a layer in which a luminescent material having apeak at 600 nm to 680 nm in an emission spectrum is dispersedly includedin a layer formed of a material (second host) that has a larger energygap than the luminescent material. As the second host, α-NPD, CBP, TCTA,Znpp₂, Zn(BOX)₂, Alq₃, or the like can be used. Further, it ispreferable that the separation layer 764 be formed so that energygenerated in the first light emitting layer 763, the second lightemitting layer 765, or the separation layer 764 can transfer to both thefirst light emitting layer 763 and the second light emitting layer 765,and be formed to have a function for preventing energy from transferringonly one of the first light emitting layer 763 and the second lightemitting layer 765. Specifically, the separation layer 764 can be formedusing a organic carrier transport material such as α-NPD, CBP, TCTA,Znpp₂, Zn(BOX)₂, or the like. As described above, by providing theseparation layer 764, the malfunction making it impossible to obtainwhite light, caused since emission strength of only one of the firstlight emitting layer 763 and the second light emitting layer 765 becomesstronger, can be prevented.

In the present embodiment mode, the luminescent material contained ineach of the first light emitting layer 763 and the second light emittinglayer 765 is not particularly limited. The luminescent materialscontained in the first light emitting layer 763 and the second lightemitting layer 765 can be interchanged; accordingly, the first lightemitting layer 763 may contain a luminescent material that is capable ofexhibiting luminescence of the longer wavelength, and the luminescentmaterial that is capable of exhibiting luminescence of the shorterwavelength is contained in the second light emitting layer 765 instead.In this case, a luminescent material which easily traps carriers is usedfor a light emitting layer (the first light emitting layer 763) that iscloser to an electrode serving as a cathode (the first electrode 751),the luminescent material included in each layer is made to emit lightmore efficiently.

In addition, in the present embodiment mode, the light emitting elementprovided with the two light emitting layers as shown in FIG. 4 isdescribed. However, the number of light emitting layers is not limitedto two, and for example, three light emitting layers may be used.Further, luminescence from each light emitting layer may be combined tobe visually recognized as white light.

Furthermore, an electron transport layer 762 may be provided between thefirst light emitting layer 763 and the first electrode 751 as shown inFIG. 4. An electron injection layer 761 may be provided between theelectron transport layer 762 and the first electrode 751 in addition tothe electron transport layer 762. Further, a hole transport layer 766may be provided between the second light emitting layer 765 and thesecond electrode 752 as shown in FIG. 4. Still further, a hole injectionlayer 767 may be provided between the hole transport layer 766 and thesecond electrode 752.

Other than the light emitting element described with reference to FIG.4, a light emitting element shown in FIG. 5 may be used.

The light emitting element shown in FIG. 5 has a first light emittinglayer 783 and a second light emitting layer 788 between a firstelectrode 771 and a second electrode 772. Between the first lightemitting layer 783 and the second light emitting layer 788, a firstlayer 785 and a second layer 786 are provided.

The first layer 785 is a layer that generates holes, and the secondlayer 786 is a layer that generates electrons. When voltage is appliedso that the potential of the second electrode 772 is higher than thepotential of the first electrode 771, an electron injected from thefirst electrode 771 and a hole injected from the first layer 785 arerecombined in the first light emitting layer 783, and a luminescentmaterial included in the first light emitting layer 783 emits light.Further, a hole injected from the second electrode 772 and an electroninjected from the second layer 786 are recombined in the second lightemitting layer 788, and a luminescent material included in the secondlight emitting layer 788 emits light.

A luminescent material having a peak at 600 nm to 680 nm in an emissionspectrum as in Embodiment Mode 2 is contained in the first lightemitting layer 783. In addition, the second light emitting layer 788includes a luminescent material typified by a fluorescent material suchas perylene, TBP, DPVBi, BCzVBi, BAlq, or Gamq₂Cl, or a phosphorescentmaterial such as Ir(CF₃ ppy)₂(pic), FIr(acac), or FIr(pic), from whichluminescence with a peak at 450 to 510 nm in an emission spectrum can beobtained. Luminescence from the first light emitting layer 783 and thesecond light emitting layer 788 is emitted from one or both of the firstelectrode 771 and the second electrode 772. Then, the luminescence fromeach light emitting layer is visually mixed to be visually recognized aswhite light.

In each of the first light emitting layer 783 and the second lightemitting layer 788, it is preferable that the luminescent material bedispersedly contained in a host.

It is preferable that the first layer 785 mainly contains a firstmaterial which transports more holes than electrons and further containsa second material having electron-acceptor properties to the firstmaterial. As the first material, the same material as the material thatis used for forming a hole transport layer may be used. In addition, asthe second material, such as molybdenum oxide, vanadium oxide,7,7,8,8-tetracyanoquinodimethane (abbreviation: TCNQ),2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (abbreviation:F4-TCNQ) can be used.

It is preferable that the second layer 786 mainly contains a thirdmaterial which transports more electrons than holes and further containsa fourth material having electron-donor properties to the thirdmaterial. As the third material, the same material as a material that isused for forming an electron transport layer may be used. In addition,as the fourth material, an alkali metal such as lithium or cesium, analkaline-earth metal such as magnesium or calcium, a rare-earth metalsuch as erbium or ytterbium, or the like can be used.

Furthermore, an electron transport layer 782 may be provided between thefirst light emitting layer 783 and the first electrode 771 as shown inFIG. 5, an electron injection layer 781 may be provided between theelectron transport layer 782 and the first electrode 771, a holetransport layer 784 may be provided between the first light emittinglayer 783 and the first layer 785. A hole transport layer 789 may beprovided between the second light emitting layer 788 and the secondelectrode 772, and a hole injection layer 790 may be provided betweenthe hole transport layer 789 and the second electrode 772. An electrontransport layer 787 may be provided between the second light emittinglayer 788 and the second layer 786.

In addition, in the present embodiment mode, the light emitting elementprovided with the two light emitting layers as shown in FIG. 5 isdescribed. However, the number of light emitting layers is not limitedto two, and for example, three light emitting layers may be used.Further, luminescence from each light emitting layer may be combined soas to be visually recognized as white light.

Embodiment Mode 4

In the present embodiment mode, a mode of a structure of a lightemitting device according to the present invention will be describedwith reference to FIGS. 6, 7A, and 7B.

FIG. 6 is a top view showing a part of a light emitting portion of alight emitting device according to the invention. In FIG. 6, a firstelectrode 502 a, a first electrode 502 b, a first electrode 502 c, and afirst electrode 502 d are sequentially arranged in rows. The firstelectrode 502 a, 502 b, 502 c, and 502 d are partially overlapped withrespective light emitting layers 506 a, 506 b, 506 c, and 506 d andsecond electrodes 507 a, 507 b, 507 c, and 507 d. Further, a part of thefirst electrode 502 a and a part of the second electrode 507 b areoverlapped, and a part of the first electrode 502 b and a part of thesecond electrode 507 c are overlapped. Still further, a part of thefirst electrode 502 c and a part of the second electrode 507 d areoverlapped. Here, one of the first electrode and the second electrodeserves as an anode and the other serves as a cathode.

FIG. 7A is a cross-sectional view showing a part of a light emittingportion of a light emitting device according to the present invention,and specifically is a cross-sectional view showing a part taken alongline A-A′ in FIG. 6. In FIG. 7A, a first electrode 502 b is providedover the substrate 501 so as to be exposed in an opening of a partitionlayer 505. A light emitting layer 506 b is provided over the firstelectrode 502 b. A second electrode 507 b is further provided over thelight emitting layer 506 b. Thus, a plurality of light emitting elementseach having a light emitting layer between a pair of electrodes areindependently provided in a part shown by the broken line A-A′.

FIG. 7B is also a cross-sectional view showing a part of a lightemitting portion of a light emitting device according to the presentinvention, and specifically is a cross-sectional view showing a parttaken along line B-B′ in FIG. 6. A substrate 501 in FIG. 7B is the sameas a substrate 501 in FIG. 7A. A first electrode 502 b in FIG. 7B is thesame as the first electrode 502 b in FIG. 7A. Other than the firstelectrode 502 b, a first electrode 502 c is provided so as to be exposedin an opening of the partition layer 505.

The first electrode 502 b and the first electrode 502 c are respectivelyexposed in two openings. In a first opening, the light emitting layer506 b is provided over the first electrode 502 b, and a light emittinglayer 506 c is provided over the first electrode 502 c. Further, asecond electrode 507 c is provided over the first electrode 502 c thatis exposed in the first opening with the light emitting layer 506 ctherebetween. A part where the first electrode 502 b, the light emittinglayer 506 b, and the second electrode 507 b are overlapped serves as onelight emitting element. A part where the first electrode 502 c, thelight emitting layer 506 c, and the second electrode 507 c areoverlapped serves as another light emitting element. The secondelectrode 507 c is overlapped with and electrically connected to thefirst electrode 502 b in the second opening. As described above, thefirst electrode included in a first light emitting element is connectedto the second electrode included in a second light emitting element;thus, a plurality of light emitting elements each having a lightemitting layer between a pair of electrodes are connected in series.

Further, a light emitting element having a structure as shown in FIG. 7Bcan be manufacture through such a process described below. First, thefirst electrodes 502 a, 502 b, 502 c, and 502 d are formed respectively,and the partition layer is thereafter formed so that each firstelectrode is exposed in two openings. Light emitting layers 506 a, 506b, 506 c, and 506 d are respectively formed over the respective firstelectrodes each exposed in one of the openings. At this point of time,the light emitting layers 506 a, 506 b, 506 c, and 506 d are preferablyformed by a formation method such as an ink-jet method or vapordeposition using a mask, by which a layer can be selectively formed at adesired position. Further, each of the second electrodes 507 a, 507 b,507 c, and 507 d is formed so as to cover the light emitting layer ofone of the light emitting elements and the first electrode exposed inthe opening of another light emitting element adjacent to the lightemitting element. In addition, it is preferable that the secondelectrodes 507 a, 507 b, 507 c, and 507 d are also formed by a formationmethod such as an ink-jet method or vapor deposition using a mask, bywhich a layer can be selectively formed at a desired position as well asthe light emitting layers. In this manner, serially connected lightemitting elements can be manufactured easily.

Embodiment Mode 5

As shown in FIG. 8, a light emitting device of the present inventionincludes a support body 6001 and a lighting portion 6002 providedthereover. Further, the light emitting device is equipped with aflexible printed circuit 6003 for connecting the lighting portion 6002and a power supply. Modes of electronic devices each including such alight emitting device of the invention will be described.

FIG. 9A shows a lighting device to which a light emitting device of theinvention is applied. In the lighting device of FIG. 9A, the lightemitting device of the invention is mounted in a frame 5700. As to thelighting device in which such a lighting device of the invention isapplied to the lighting portion, malfunction due to a defect of a lightemitting element can be reduced and good lighting can be performed.

FIG. 9B shows a personal computer in which a light emitting device ofthe invention is applied to a backlight in a display area 5523. Thepersonal computer includes a main body 5521, a frame 5522, the displayarea 5523, a keyboard 5524, and the like. Specifically, a liquid crystaldevice 902 and a backlight 903 is fit between a frame 901 and a frame904 to be mounted on the personal computer. The liquid crystal displaydevice is operated in accordance with a signal supplied via a drivercircuit 905. The backlight 903 is equipped with a flexible printedcircuit 906. A signal for controlling the backlight 903 is inputted tothe backlight via the flexible printed circuit 906. A light emittingdevice of the invention is mounted in this manner; thus, malfunctionsuch as a dark spot locally formed due to a defect of the light emittingelement can be reduced and good display can be performed.

1. A lighting device comprising: a plurality of circuits connected inparallel with each other, each of the plurality of circuits comprising:a first light emitting element and a second light emitting element whichare connected in series, each comprising: a first electrode; a firstlight emitting layer over the first electrode; a second light emittinglayer over the first light emitting layer; a separation layer comprisingan organic carrier transportation material between the first lightemitting layer and the second light emitting layer; and a secondelectrode over the second light emitting layer, wherein the firstelectrode of the first light emitting element and the first electrode ofthe second light emitting element are separated from each other.
 2. Thelighting device according to claim 1, further comprising a partitionlayer covering an edge portion of the first electrodes of the firstlight emitting element and the second light emitting element, whereinthe partition layer includes a first opening over the first electrode ofthe first light emitting element and a second opening over the firstelectrode of the second light emitting element.
 3. The lighting deviceaccording to claim 1, wherein a wavelength of light from the first lightemitting layer is different from a wavelength of light from the secondlight emitting layer so that light emitting from the lighting device iswhite light.
 4. The lighting device according to claim 1, wherein atleast one of the first light emitting layer and the second lightemitting layer comprises a phosphorescent material.
 5. The lightingdevice according to claim 1, wherein one of the first light emittinglayer and the second light emitting layer emits light having awavelength range at 450 to 510 nm.
 6. The lighting device according toclaim 1, wherein one of the first light emitting layer and the secondlight emitting layer emits light having a wavelength range at 600 to 680nm.
 7. The lighting device according to claim 1, wherein each of thefirst light emitting element and the second light emitting elementcomprises a third light emitting layer.
 8. The lighting device accordingto claim 1, wherein each of the first light emitting element and thesecond light emitting element comprises a layer which is between thefirst light emitting layer and the first electrode, and the layerincludes a metal oxide.
 9. The lighting device according to claim 8,wherein the metal oxide comprises a material selected from the groupconsisting of MoOx, VOx, RuOx, WOx, and MnOx.
 10. The lighting deviceaccording to claim 1, wherein the first light emitting layer of thefirst light emitting element and the first light emitting layer of thesecond light emitting element are spaced from each other.
 11. Thelighting device according to claim 1, wherein the first light emittinglayer of the first light emitting element of the plurality of circuitsis continuous to the first light emitting layer of adjacent one of thefirst light emitting element of the plurality of circuits, and whereinthe first light emitting layer of the second light emitting element ofthe plurality of circuits is continuous to the first light emittinglayer of adjacent one of the second light emitting element of theplurality of circuits.
 12. The lighting device according to claim 1,wherein the second electrode of the first light emitting element isconnected to the first electrode of the second light emitting element.13. The lighting device according to claim 1, wherein the plurality ofcircuits each are arranged to be applied a voltage simultaneously. 14.The lighting device according to claim 1, wherein the first lightemitting element and the second light emitting element are arranged toemit light simultaneously.
 15. A lighting device comprising: a pluralityof circuits connected in parallel with each other, each of the pluralityof circuits comprising: a first light emitting element and a secondlight emitting element which are connected in series, each comprising: afirst electrode; a first light emitting layer over the first electrode;a second light emitting layer over the first light emitting layer; aseparation layer capable of transferring an energy generated in one ofthe first light emitting element and the second light emitting elementto the other; and a second electrode over the second light emittinglayer, wherein the first electrode of the first light emitting elementand the first electrode of the second light emitting element areseparated from each other.
 16. The lighting device according to claim15, further comprising a partition layer covering an edge portion of thefirst electrodes of the first light emitting element and the secondlight emitting element, wherein the partition layer includes a firstopening over the first electrode of the first light emitting element anda second opening over the first electrode of the second light emittingelement.
 17. The lighting device according to claim 15, wherein awavelength of light from the first light emitting layer is differentfrom a wavelength of light from the second light emitting layer so thatlight emitting from the lighting device is white light.
 18. The lightingdevice according to claim 15, wherein at least one of the first lightemitting layer and the second light emitting layer comprises aphosphorescent material.
 19. The lighting device according to claim 15,wherein one of the first light emitting layer and the second lightemitting layer emits light having a wavelength range at 450 to 510 nm.20. The lighting device according to claim 15, wherein one of the firstlight emitting layer and the second light emitting layer emits lighthaving a wavelength range at 600 to 680 nm.
 21. The lighting deviceaccording to claim 15, wherein each of the first light emitting elementand the second light emitting element comprises a third light emittinglayer.
 22. The lighting device according to claim 15, wherein each ofthe first light emitting element and the second light emitting elementcomprises a layer which is between the first light emitting layer andthe first electrode, and the layer includes a metal oxide.
 23. Thelighting device according to claim 22, wherein the metal oxide comprisesa material selected from the group consisting of MoOx, VOx, RuOx, WOx,and MnOx.
 24. The lighting device according to claim 15, wherein thefirst light emitting layer of the first light emitting element and thefirst light emitting layer of the second light emitting element arespaced from each other.
 25. The lighting device according to claim 15,wherein the first light emitting layer of the first light emittingelement of the plurality of circuits is continuous to the first lightemitting layer of adjacent one of the first light emitting element ofthe plurality of circuits, and wherein the first light emitting layer ofthe second light emitting element of the plurality of circuits iscontinuous to the first light emitting layer of adjacent one of thesecond light emitting element of the plurality of circuits.
 26. Thelighting device according to claim 15, wherein the second electrode ofthe first light emitting element is connected to the first electrode ofthe second light emitting element.
 27. The lighting device according toclaim 15, wherein the plurality of circuits each are arranged to beapplied a voltage simultaneously.
 28. The lighting device according toclaim 15, wherein the first light emitting element and the second lightemitting element are arranged to emit light simultaneously.
 29. Alighting device comprising: a plurality of circuits connected inparallel with each other, each of the plurality of circuits comprising:a first light emitting element and a second light emitting element whichare connected in series, each comprising: a first electrode; a firstlight emitting layer over the first electrode; a second light emittinglayer over the first light emitting layer; a separation layer capable ofallowing each of the first light emitting layer and the second lightemitting layer to emit light to obtain white light emission from thelighting device; and a second electrode over the second light emittinglayer, wherein the first electrode of the first light emitting elementand the first electrode of the second light emitting element areseparated from each other.
 30. The lighting device according to claim29, further comprising a partition layer covering an edge portion of thefirst electrodes of the first light emitting element and the secondlight emitting element, wherein the partition layer includes a firstopening over the first electrode of the first light emitting element anda second opening over the first electrode of the second light emittingelement.
 31. The lighting device according to claim 29, wherein awavelength of light from the first light emitting layer is differentfrom a wavelength of light from the second light emitting layer so thatlight emitting from the lighting device is white light.
 32. The lightingdevice according to claim 29, wherein at least one of the first lightemitting layer and the second light emitting layer comprises aphosphorescent material.
 33. The lighting device according to claim 29,wherein one of the first light emitting layer and the second lightemitting layer emits light having a wavelength range at 450 to 510 nm.34. The lighting device according to claim 29, wherein one of the firstlight emitting layer and the second light emitting layer emits lighthaving a wavelength range at 600 to 680 nm.
 35. The lighting deviceaccording to claim 29, wherein each of the first light emitting elementand the second light emitting element comprises a third light emittinglayer.
 36. The lighting device according to claim 29, wherein each ofthe first light emitting element and the second light emitting elementcomprises a layer which is between the first light emitting layer andthe first electrode, and the layer includes a metal oxide.
 37. Thelighting device according to claim 36, wherein the metal oxide comprisesa material selected from the group consisting of MoOx, VOx, RuOx, WOx,and MnOx.
 38. The lighting device according to claim 29, wherein thefirst light emitting layer of the first light emitting element and thefirst light emitting layer of the second light emitting element arespaced from each other.
 39. The lighting device according to claim 29,wherein the first light emitting layer of the first light emittingelement of the plurality of circuits is continuous to the first lightemitting layer of adjacent one of the first light emitting element ofthe plurality of circuits, and wherein the first light emitting layer ofthe second light emitting element of the plurality of circuits iscontinuous to the first light emitting layer of adjacent one of thesecond light emitting element of the plurality of circuits.
 40. Thelighting device according to claim 29, wherein the second electrode ofthe first light emitting element is connected to the first electrode ofthe second light emitting element.
 41. The lighting device according toclaim 29, wherein the plurality of circuits each are arranged to beapplied a voltage simultaneously.
 42. The lighting device according toclaim 29, wherein the first light emitting element and the second lightemitting element are arranged to emit light simultaneously.
 43. Alighting device comprising: a plurality of circuits connected inparallel with each other, each of the plurality of circuits comprising:a first light emitting element and a second light emitting element whichare connected in series, each comprising: a first electrode; a firstlight emitting layer over the first electrode; a second light emittinglayer over the first light emitting layer; and a second electrode overthe second light emitting layer, wherein the first electrode of thefirst light emitting element and the first electrode of the second lightemitting element are separated from each other.
 44. The lighting deviceaccording to claim 43, further comprising a partition layer covering anedge portion of the first electrodes of the first light emitting elementand the second light emitting element, wherein the partition layerincludes a first opening over the first electrode of the first lightemitting element and a second opening over the first electrode of thesecond light emitting element.
 45. The lighting device according toclaim 43, wherein a wavelength of light from the first light emittinglayer is different from a wavelength of light from the second lightemitting layer so that light emitting from the lighting device is whitelight.
 46. The lighting device according to claim 43, wherein at leastone of the first light emitting layer and the second light emittinglayer comprises a phosphorescent material.
 47. The lighting deviceaccording to claim 43, wherein one of the first light emitting layer andthe second light emitting layer emits light having a wavelength range at450 to 510 nm.
 48. The lighting device according to claim 43, whereinone of the first light emitting layer and the second light emittinglayer emits light having a wavelength range at 600 to 680 nm.
 49. Thelighting device according to claim 43, wherein each of the first lightemitting element and the second light emitting element comprises a thirdlight emitting layer.
 50. The lighting device according to claim 43,wherein each of the first light emitting element and the second lightemitting element comprises a layer which is between the first lightemitting layer and the first electrode, and the layer includes a metaloxide.
 51. The lighting device according to claim 50, wherein the metaloxide comprises a material selected from the group consisting of MoOx,VOx, RuOx, WOx, and MnOx.
 52. The lighting device according to claim 43,wherein the first light emitting layer of the first light emittingelement and the first light emitting layer of the second light emittingelement are spaced from each other.
 53. The lighting device according toclaim 43, wherein the first light emitting layer of the first lightemitting element of the plurality of circuits is continuous to the firstlight emitting layer of adjacent one of the first light emitting elementof the plurality of circuits, and wherein the first light emitting layerof the second light emitting element of the plurality of circuits iscontinuous to the first light emitting layer of adjacent one of thesecond light emitting element of the plurality of circuits.
 54. Thelighting device according to claim 43, wherein the second electrode ofthe first light emitting element is connected to the first electrode ofthe second light emitting element.
 55. The lighting device according toclaim 43, wherein the plurality of circuits each are arranged to beapplied a voltage simultaneously.
 56. The lighting device according toclaim 43, wherein the first light emitting element and the second lightemitting element are arranged to emit light simultaneously.